Equatorial Circulation

• Subtle changes in winds give rise to

complicated surface current patterns

• Equatorial Undercurrent

• Focus on Pacific circulation, Atlantic

patterns are similar. Indian ocean ones

are not...

• Transient behavior - El Nino/Southern

Oscillation

Equatorial Currents

Munk’s Solution

Wind-Driven Equatorial Circulation

Wind-Driven Equatorial Circulation

Equatorial Surface Currents

Wind-Driven Equatorial Circulation

Wind-Driven Equatorial Circulation

Equatorial Convergence

Photographs of the convergent front from R/V Thompson, taken by Edward Peltzer

.

Equatorial Convergence

Photograph from the Space Shuttle Atlantis, at an altitude of 230 km on 7 August, 1992. Distance scale is ~ 100 km.

Equatorial Surface Currents

• Subtle changes in wind stress give

rise to a complicated set of currents

– South Equatorial Current (SEC)

– North Equatorial Counter Current (NECC)

– North Equatorial Current (NEC)

• Vertical circulation cells are also

observed

Equatorial Divergence

• Westward trades drive a divergence

of Ekman transports from equator

• Results in upwelling of cool water

• Strongest in the eastern portion

of Pacific

• The cool tongue

Equatorial Divergence

Equatorial Divergence

Equatorial Divergence

Ocean Biogeochemistry in a Nutshell

• Light energy drives the net fixation of carbon

• Within the euphotic zone, nutrients & CO2 produce

CO2 & fixed carbon

• Below the euphotic zone, the rxn’s reverse

PelagicEcosystems

NUTS

CO2

Fixed Carbon

O2

h

Equatorial Divergence

Equatorial Divergence

Equatorial Divergence

The Equatorial Undercurrent

• West wind drift piles up water in

western equatorial Pacific

• Creates a return subsurface flow

that is locked to the equator

• Rises as a goes across the

Pacific

• Also found in Atlantic

Equatorial Undercurrents

• West wind drift piles up water in

western equatorial Pacific

• Creates a subsurface return flow

locked to the equator

• EUC flow rates are huge ~100 cm/s!!

• EUC rises due to equatorial divergence

• A transient feature

Equatorial Undercurrent

Equatorial Undercurrent

EUC @ 150W

EUC @ 155W

ADCP Observatio

ns

ADCP Observations

Frequency Range Resolution 38 kHz 800-1000m 24m 75 kHz 560-700m 16m 150 kHz 380-425m 8m

ADCP Principles

• Acoustic Doppler Current Profiler (ADCP)

• Transmits a succession of acoustic pulses

• Segments backscatter echoes into depth

cells

• Doppler shift in carrier frequency

determines along beam velocities

• Reconciling four beams - provides estimate

of horizontal current

Direct Observations

Johnson et al.[2002]

Direct Observations

ADCP velocity sections across Pacific

NECCEUC

SEC

EUC

Direct Observations

Temperature sections across Pacific

Transport

Mid-Basin Transport EUC - 30 Sv NECC - 20 Sv SEC - 50 Sv

Peak month EUC - March NECC - December SEC(N) - April SEC(S) - February

NECC

EUC

SEC(N)

SEC(S)

Current

Location

Mid-Basin Transport EUC - On equator NECC - 4 to 6oN

NECC

EUC

SEC(N)

SEC(S)

Review

• Subtle changes in wind stress give

rise to

– South Equatorial Current (SEC)

– North Equatorial Counter Current (NECC)

– North Equatorial Current (NEC)

• Equatorial undercurrents are often

found

• Transient behavior is important (ENSO)